Bistability is a well known phenomenon, and the bistable behavior of a number of magnetic materials has been long since exploited in electronics in order to manufacture logic devices, in particular memory devices. The phenomenon is characterized by the existence of two values of an output quantity in correspondence with a given value of an input variable, the attainment of either output value depending on the direction in which the input value is made to vary.
More recently the same phenomenon has been observed in optical devices (interferometers) made of materials with non-linear properties, i.e. materials in which certain intrinsic parameters (such as refractive index and absorption constant) depend on the input power. More particularly, the refractive index of these devices can be expressed as the sum of a constant term and of a term depending on the power I of the signal sent to the device, according to the relation EQU n=n.sub.0 +n.sub.2 .multidot.I
where n.sub.0 is the linear refractive index (which is constant), while n.sub.2 is the so-called non-linear refractive index coefficient.
Owing to the present interest in optical communication systems, which allow much higher processing speeds than electronic components, it has been proposed to exploit optical bistability to implement optical logic circuits capable of replacing as mush as possible the electronic components in these systems. Optical memory devices to be used, e.g. in optical switching and processing, have been widely described in the literature; these devices use semiconductor lasers (or laser diodes), excited by optical or electrical pulses and caused to operate near the stimulated emission threshold. Under such conditions, in fact, the laser operates as an amplifier and has an optical power threshold, for switching from the spontaneous emission condition to the stimulated emission condition, different from the threshold which restores the device to the spontaneous emission condition. The existence of two switching thresholds gives rise to the bistable behavior of the device.
For a correct use of a laser under the conditions above, its characterization from bistability standpoint is also necessary and, more particularly, the non-linear refractive index coefficient should be determined. Various methods are known for measuring this coefficient in the non-linear material from which the device will be made. The simplest method is based on interferometric techniques and is described, e.g. by D. Milam and M. J. Weber in the paper entitiled "Measurement of non-linear refractive index coefficient using time-resolved interferometry: Applications to optical materials for high-power neodymium lasers", Journal of Applied Physics, Vol. 47, 1976, pages 2497 ff.
According to this method, a sample of the material is introduced into an interferometer branch, a variable-intensity light beam is launched into the sample, so as to cause a refractive index variation, and the interference fringe shifts due to the refractive index variations are measured: n.sub.2 is obtained from such shifts. A correct evaluation of the positions of the visibility maxima and minima requires an accurate digital processing of the experimental data to eliminate the noise present in the measurement.
No technique has, until now, been suggested for measuring n.sub.2 directly under a device in operating conditions, This determination can be more significant than a determination made on the material, since it must be presumed that the non-linear refractive index coefficient, like the linear refractive index, is modified when the material is introduced into a device.